Uricase was partially purified from livers of five mammals (dog, bovine, horse, house musk shrew and guinea pig), and its immunological properties were studied using anti-hog liver uricase prepared in rabbits. 2. Results obtained by immunodiffusion suggest that hog uricase has four sites of partial antigens (a, b, c, d), while the enzyme of other mammals has only part of them, and a cladogram of these six mammals is presented showing development of partial antigens. 3. Results obtained by immunoelectrophoresis suggest that the protein structure of house musk shrew uricase is considerably different from the other five mammals tested.